Click the title of a thread in the forum to see a list of all the messages associated with it.
Click Show Options
on the far right above the messages or click the down arrow 
to display the option buttons.
Using course tools>>Discussion Board>>Participating in forums
To remove one or more messages from a Discussion Board forum:
Click the title of a thread in the forum to see a list of all the messages associated with it.
Click Show Options
on the far right above the messages or click the down arrow to display the option buttons.
Do one of the following:
Select each message you want to remove by
clicking the box 
to the left of it.
Select all messages by clicking Select All.
Collect the messages you want to delete by
clicking the boxes to the left of them, then click Collect.
Click Remove.
Click OK to confirm the deletion.
Tip: Instead of deleting forum messages, you may want to save the forum in an archive for future reference.
Caution: If you delete a message by mistake, you must create it again from scratch.